Semiconductor device and method for fabricating the same

ABSTRACT

Described is a method for fabricating a semiconductor device having an FET of a trench-gate structure obtained by disposing a conductive layer, which will be a gate, in a trench extended in the main surface of a semiconductor substrate, wherein the upper surface of the trench-gate conductive layer is formed higher than the main surface of the semiconductor substrate and the trench gate conductive layer and gate insulating film are formed in the trench and over the main surface of the semiconductor substrate at the periphery of the trench. In this method, a trench wherein a trench-gate is to be formed is formed on the main surface of the semiconductor substrate with the insulating film formed thereon with a mask; and the side surface of the insulating film is caused to retreat from the upper end of the trench by isotropic etching, whereby a gate insulating film and a conductive layer to be the trench gate are formed in the trench and over the main surface of the semiconductor substrate at the periphery of the trench. According to the present invention, occurrence of a source offset and damage of a gate insulating film can be prevented.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a semiconductor device, particularly, a technique effective when adapted to a semiconductor device having a trench-gate structure.

[0002] A power transistor has been used for various applications including a power amplifier circuit, power supply circuit, converter and power supply protective circuit. Since it treats high power, it is required to have high breakdown voltage and to permit high current.

[0003] In the case of MISFET (Metal Insulator Semiconductor Field Effect Transistor), high current can be attained easily by an expansion of a channel width. In order to avoid an increase in a chip area caused by such expansion of a channel width, a mesh-gate structure is, for example, employed.

[0004] Gates are two-dimensionally arranged in the form of a lattice in the mesh-gate structure so that a channel width per unit chip area can be enlarged. A description of an FET having a mesh-gate structure can be found on pages 429 to 430 of “Semiconductor Handbook” published by Ohmsha Limited or U.S. Pat. No. 5,940,721.

[0005] For such a power FET, a planar structure has conventionally been employed because its fabrication process is simple and an oxide film which will serve as a gate insulating film can be formed easily. In the above-described U.S. Pat. No. 5,940,721, shown is an FET having a planar structure.

[0006] The FET having a planar structure is however accompanied with the drawbacks that when a gate is formed narrowly, a channel length becomes short and a short-channel effect appears' because the channel length is determined depending on the gate length; or when a gate is formed narrowly, an allowable current decreases because the gate has additionally a function of interconnection. It is therefore impossible to conduct miniaturization freely. With the foregoing in view, adoption of an FET having a trench-gate structure is considered because it can improve the integration degree of cells and in addition, reduce an on resistance.

[0007] The trench-gate structure is formed by disposing, via an insulating film, a conductive layer, which will serve as a gate, in a trench extended in the main surface of a semiconductor substrate and in this structure, the deeper portion and the outer surface portion of the main surface serve as a drain region and a source region, respectively and a semiconductor layer between the drain and source regions serves as a channel forming region.

[0008] An MISFET having such a trench-gate structure is disclosed, for example, in Japanese Patent Application Laid-Open No. Hei 8-23092 or U.S. Pat. No. 5,918,114.

SUMMARY OF THE INVENTION

[0009] With an advance of the miniaturization of a device, there is a tendency to make the source region shallower. When the source region becomes shallower, however, it becomes difficult to place a trench gate at a precise position and the end portion of the trench gate does not overlap with the source region. If a source offset occurs, in other words, the trench gate gets out of the source region, by inaccurate positioning of the trench gate, this source offset impairs the functioning of the FET.

[0010] Since the end portion of the gate insulating film is positioned at the corner of the trench, it is sometimes damaged during the formation of a trench gate. Such an inferior gate insulating film happens to cause operation failure of the device.

[0011] An object of the present invention is to provide a technique capable of overcoming the above-described problems and preventing the occurrence of a source offset.

[0012] Another object of the present invention is to provide a technique capable of overcoming the above-described problems and preventing the damage of a gate insulating film.

[0013] A further object of the present invention is to provide a FET having a thinned trench-gate structure.

[0014] The above-described and the other objects and novel features of the present invention will be apparent from the description herein and accompanying drawings.

[0015] Among the inventions disclosed by the present application, representative ones will next be summarized simply.

[0016] In a semiconductor device equipped with an FET having a trench-gate structure obtained by disposing a conductive layer, which will be a gate, in a trench extended in the main surface of a semiconductor substrate, a trench-gate conductive layer (gate electrode) and a gate insulating film are formed in the trench and over the main surface of the semiconductor substrate at the periphery of the trench.

[0017] A fabrication method of the above-described semiconductor device comprises forming an insulating film over the main surface of the semiconductor substrate, patterning the insulating film according to a pattern corresponding to the trench-gate, that is, gate electrode, forming a trench, wherein a trench-gate will be formed, in the semiconductor substrate with the patterned insulating film as a mask, etching the side surface of the insulating film by isotropic etching to cause it retreat from the upper end portion of the trench, forming a gate insulating film and a conductive layer, which will be a trench gate, in the trench and over the main surface of the semiconductor substrate at the periphery of the trench, and then forming a channel forming region and a source region to be brought into contact with the gate insulating film in the trench.

[0018] In the above-described means, the upper surface of the trench-gate conductive layer is formed higher than the main surface of the semiconductor substrate, which makes it possible to prevent occurrence of a source offset. In addition, a gate insulating film and a conductive film which will be a trench gate are formed over the main surface of the semiconductor substrate at the periphery of the trench so that it is possible to prevent the damage at the end portion of the gate insulating film.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a plan view illustrating the semiconductor device according to Embodiment 1 of the present invention;

[0020]FIG. 2 is an equivalent circuit view of the semiconductor device according to Embodiment 1 of the present invention;

[0021]FIG. 3 is a fragmentary plan view illustrating the semiconductor device according to Embodiment 1 of the present invention;

[0022]FIG. 4 is a partial longitudinal cross-sectional view taken along a line a-a of FIG. 3;

[0023]FIG. 5 is a partial longitudinal cross-sectional view illustrating a protecting diode of the semiconductor device according to Embodiment 1 of the present invention;

[0024]FIG. 6 is a longitudinal fragmentary cross-sectional view illustrating, in the order of steps, the semiconductor device according to Embodiment 1 of the present invention;

[0025]FIG. 7 is a longitudinal cross-sectional view illustrating, in the order of steps, a trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0026]FIG. 8 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0027]FIG. 9 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0028]FIG. 10 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0029]FIG. 11 is a longitudinal fragmentary cross-sectional view illustrating the semiconductor device according to Embodiment 1 of the present invention;

[0030]FIG. 12 is a longitudinal fragmentary cross-sectional view illustrating the semiconductor device according to Embodiment 1 of the present invention;

[0031]FIG. 13 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0032]FIG. 14 is a longitudinal fragmentary cross-sectional view illustrating, in the order of steps, the semiconductor device according to Embodiment 1 of the present invention;

[0033]FIG. 15 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0034]FIG. 16 is a longitudinal fragmentary cross-sectional view illustrating, in the order of steps, the semiconductor device according to Embodiment 1 of the present invention;

[0035]FIG. 17 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0036]FIG. 18 is a longitudinal fragmentary cross-sectional view illustrating, in the order of steps, the semiconductor device according to Embodiment 1 of the present invention;

[0037]FIG. 19 is a longitudinal fragmentary cross-sectional view illustrating, in the order of steps, the semiconductor device according to Embodiment 1 of the present invention;

[0038]FIG. 20 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0039]FIG. 21 is a longitudinal fragmentary cross-sectional view illustrating, in the order of steps, the semiconductor device according to Embodiment 1 of the present invention;

[0040]FIG. 22 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0041]FIG. 23 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0042]FIG. 24 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention;

[0043]FIG. 25 is a longitudinal cross-sectional view illustrating, in the order of steps, the trench gate of the semiconductor device according to Embodiment 1 of the present invention; and

[0044]FIG. 26 is a longitudinal cross-sectional view illustrating, in the order of steps, the semiconductor device according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] The embodiments of the present invention will hereinafter be described. In all the drawings for describing the embodiments, like members of a function will be identified by like reference numerals and overlapping descriptions will be omitted.

[0046] (Embodiment 1 )

[0047]FIG. 1 is a plan view illustrating a power MISFET having a trench-gate structure, which will be an essential part of the semiconductor device according to Embodiment 1 of the present invention, while FIG. 2 is an equivalent circuit view of the MISFET shown in FIG. 1. FIG. 3 is a fragmentary plan view illustrating the portion a of FIG. 1 in an enlarged scale and FIG. 4 is a longitudinal cross-sectional view taken along a line a-a of FIG. 3.

[0048] The MISFET of this embodiment is formed on a semiconductor substrate obtained, for example, by forming, by epitaxial growth, an epitaxial layer 2 on an n⁺-type semiconductor body made of single crystal silicon. This MISFET is disposed in the form of a rectangular ring along the outer periphery of the semiconductor substrate and it is formed within a region surrounded by a plate-shaped field insulating film 3 (shown by a double slash in FIG. 3) having a rectangular portion inside of the corner.

[0049] Within the above-described region, a plurality of hexagonal or flat pentagonal cells having a trench-gate structure are disposed regularly and a mesh-gate structure wherein gates are two-dimensionally disposed in the lattice form and cells are connected in parallel each other is formed. Semiconductor island regions of the cells are therefore separated each other in the trench-gate structure.

[0050] In each of the cells, an n⁺-type first semiconductor layer 2 a formed over a semiconductor body 1 serves as a drain region, a p-type second semiconductor layer 2 b formed over the first semiconductor layer 2 a serves as a base region wherein a channel is to be formed, and an n⁺-type third semiconductor layer 2 c formed over the semiconductor layer 2 b serves as a source region, thus forming a vertical FET.

[0051] A trench gate (gate electrode) 4 is formed, via a gate insulating film 5, in a trench which extends from the main surface of a semiconductor substrate to the n⁻-type second semiconductor layer 2 a which will be a drain region. As the trench gate 4, for example impurity-introduced polycrystalline silicon is employed, while the gate insulating film 5 is made of a multilayer film obtained, for example, by successively stacking a thermal oxide film of about 27 nm thick and a deposition film of about 50 nm thick.

[0052] As illustrated in FIGS. 19 to 21 which will be described later, the upper surface 4 a of the trench gate 4 of this embodiment is formed higher than the surface of the third semiconductor layer 2 c, which will be a source region, that is, the main surface of the semiconductor substrate. Such a constitution makes it possible to prevent the trench gate 4 from getting out of the source region, that is, prevent occurrence of a source offset even if the source region is made shallow. The trench gate 4 is desired to have a substantially flat or convex upper surface.

[0053] Over the main surface of the semiconductor substrate at the periphery of the trench, the trench gate 4 and gate insulating film 5 are also formed. Such a constitution makes it possible to prevent the failure of the gate insulating film 5.

[0054] As described above, the trench gates 4 of the adjacent cells are connected each other and the trench gate 4 of the cell positioned at the outer periphery is connected with a gate wiring 6, for example, made of polycrystalline silicon in the vicinity at the outer periphery of a semiconductor chip.

[0055] The gate wiring 6 is electrically connected with a gate guard ring 8 (partially shown by a broken line in FIG. 3) which is formed thereover through an interlayer insulating film 7 and is, for example, made of silicon-containing aluminum. The gate guard ring 8 is integrated with a rectangular gate electrode 9 (partially shown by a broken line in FIG. 3) which is disposed at the rectangular portion of the field insulating film 3. The gate electrode 9 has a connecting region (shown by a broken line in FIG. 1) with the gate 4.

[0056] A third semiconductor layer 2 c, which will be a source, is electrically connected with a source wiring 10 (partially shown by a broken line in FIG. 3) which is formed over the main surface of the semiconductor substrate through the interlayer insulating film 7 and is, for example, made of silicon-containing aluminum. The source wiring 10 has a connecting region (shown by a broken line in FIG.1) with the third semiconductor layer 2 c, which will be a source. This source wiring 10 is electrically connected with not only third semiconductor layer 2 c which will be a source but also a p⁺-type contact layer 11 disposed in the second semiconductor layer 2 b to make the base potential, that is, the potential of a channel forming region constant.

[0057] As is illustrated in FIG. 2, 3 or 4, a protective diode 12 having a back-to-back structure is disposed between the gate and source for protecting breakdown of the gate insulating film 5 from the surge from the source. FIG. 5 is a longitudinal cross-sectional view of the protective diode 12 in an enlarged scale. In the protective diode 12, the n⁺-type semiconductor regions 12 a and p-type semiconductor regions 12 b are alternately formed in a concentric ring and with the n⁺-type semiconductor regions 12 at both ends are electrically connected the gate electrode 9 and source wiring 10, respectively.

[0058] At the outer periphery of the field insulating film 3, disposed is a source guard ring 13 obtained by connecting an n⁺-type semiconductor region 13 a disposed over the main surface of the semiconductor substrate with a wiring 13 b (partially shown by a broken line in FIG. 3), for example, made of silicon-containing aluminum. Similar to the source wiring 10, the wiring 13 b of the source guard ring 13 is connected with the n⁺-type semiconductor region 12 a of the protective diode 12.

[0059] The gate wiring 6 and gate guard ring 8 are disposed over the field insulating film 3 disposed in the form of a rectangular ring, while the gate electrode 9 and protective diode 12 are disposed over the rectangular portion disposed at the corner of the field insulating film 3.

[0060] Along the field insulating film 3 in the rectangular ring form, a p-type well 14 is formed therebelow. By connecting the p-type well 14 with the terminal portion of the trench gate 4 through the gate insulating film 5, the depletion layer can be gently extended below the field insulating film 3 and discontinuity of the depletion layer can be prevented. The p-type well 14 therefore functions as a field relaxing portion for relaxing the electric field at the terminal portion of the trench gate 4.

[0061] All over the main surface of the semiconductor substrate, a protective insulating film 15 obtained, for example, by using a silicon oxide film formed by plasma CVD with tetraethoxysilane (TEOS) as a main source gas and polyimide is formed to cover the gate guard ring 8, gate electrode 9, source wiring 10 and source guard ring 13. A contact hole is made in this protective insulating film 15 to partially dispose the gate electrode 9 and source wiring 10. The gate electrode 9 and source wiring 10 thus exposed by this contact hole will become connecting regions with the gate and source, respectively. Electric connection with these connecting regions is conducted by wire bonding or the like.

[0062] As the connecting region with the drain, a drain electrode 16 which is electrically conductive with the n⁺-type semiconductor substrate 1 is formed, for example, as a nickel-titanium-nickel-silver laminated film all over the reverse side of the semiconductor substrate, and the drain electrode 16 is electrically connected with a lead frame by a conductive adhesive.

[0063] A fabrication method of the above-described semiconductor device will next be described based on FIGS. 6 to 25.

[0064] Over the n⁺-type semiconductor body 1 made of single crystal silicon having, for example, arsenic (As) introduced therein, an n⁺-type epitaxial layer 2 having a lower concentration than the semiconductor body 1 is formed to give a thickness of about 5 μm by epitaxial growth. A silicon oxide film of about 600 nm thick is then formed over the main surface of this semiconductor substrate, for example, by thermal oxidation, followed by the formation of a mask by photolithography. By etching with this mask, a plate-state field insulating film 3 is formed in the rectangular ring form along the outer periphery of the semiconductor substrate having at the inside corner thereof a rectangular portion. A mask is formed along the inner periphery of this field insulating film 3 by photolithography. With this mask, ions, for example, boron (B) are implanted and the impurities thus introduced are diffused, whereby the p-type well 14 which will be a field relaxing portion is formed as shown in FIG. 6. The impurity concentration of the p-type well 14 is, for example, set equal to or lower than that of the second semiconductor layer 2 b.

[0065] A relatively thick insulating film 17 of about 600 nm thick is then formed by thermal oxidation over the main surface of the semiconductor substrate, followed by the formation, by photolithography, of a resist mask 18 having a pattern of a trench gate for a mesh gate structure wherein gates are two-dimensionally disposed in the lattice form. By etching through this resist mask 18, a contact hole is formed according to the pattern to expose the main surface of the semiconductor substrate. The trench gate portion in this stage is illustrated in an enlarged scale in FIG. 7.

[0066] With this insulating film 17 as a mask, a trench of, for example, about 1.6 μm deep is formed by dry etching in the main surface of the semiconductor substrate as illustrated in FIG. 8. By this trench, the main surface of the semiconductor substrate is separated into plural semiconductor island regions 2 on which a cell is to be formed.

[0067] The trench thus formed by dry etching is then subjected to isotropic wet etching and chemical dry etching to relax the corner at the edge of the bottom surface of the trench and at the same time, to cause the side surface of the insulating film 17 to retreat from the upper end of the trench, as illustrated in FIG. 9.

[0068] A gate insulating film 5 is then formed by stacking a silicon oxide film of about 50 nm thick over a thermal oxide film of about 27 nm thick by CVD as illustrated in FIGS. 10 and 11.

[0069] All over the main surface of the semiconductor substrate including the inside of the trench, a polycrystalline silicon film 4′ which will be a conductive film of the trench gate 4 is formed by CVD. Into this polycrystalline silicon film 4′, impurities (ex. phosphorus) for reducing its resistance are introduced during or after deposition. The impurity concentration is set to fall within a range of 1E18/cm³ to 1E21/cm³ (1 ×10¹⁸ to 1 ×10²¹/cm³). This stage is illustrated in FIGS. 12 and 13.

[0070] The polycrystalline silicon film 4′ is removed by etching, whereby a trench gate 4 is formed in the trench. Simultaneously with this etching, a gate wiring 6 connected with the trench gate 4 and a polycrystalline silicon film 9 a which will lie under the gate electrode 9 are formed over the rectangular ring portion and the rectangular portion of the field insulating film 3, respectively, which are illustrated in FIGS. 14 and 15.

[0071] The unnecessary portion of the insulating film 17 remaining on the main surface of the semiconductor substrate is then removed, whereby the main surface of the semiconductor substrate is exposed, as illustrated in FIGS. 16 and 17.

[0072] In this stage, the insulating film 17 has been caused to retreat by the above-described isotropic etching so that the gate insulating film 5 and conductive film of the trench gate 4 have been formed on the surface of the third semiconductor layer 2 c, which will be a source region at the periphery of the trench, that is, on the main surface of the semiconductor substrate. In other words, the gate insulating film 5 and conductive film of the trench gate 4 cover the periphery of the trench as if the trench gate 4 is equipped with eaves. These eaves can prevent the damage of the gate insulating film 5 at the corner of the trench. The retreat of the insulating film 17 is conducted in self alignment so that the covering of the trench can be suppressed to the minimum size.

[0073] After formation of an insulating film 12 c made of silicon oxide, a polycrystalline silicon film is deposited over the insulating film 12 c. Into the polycrystalline silicon film, p-type impurities are then introduced, followed by patterning on the rectangular portion of the field insulating film 3 into a concentric ring around the polycrystalline silicon film 9 a on the gate electrode 9. Upon patterning, the insulating film 12 c serves as an etching stopper to prevent the trench gate 4 and gate wiring 6 from being patterned. Then, an n⁺-type semiconductor region 12 a is formed, for example, by ion implantation, whereby a protective diode 12 having the n⁺semiconductor region 12 a and p-type semiconductor region 12 b formed alternately in the concentric ring form is formed, which is illustrated in FIG. 18.

[0074] P-type impurity ions (ex. boron) are then implanted all over the surface of the epitaxial layer 2, followed by diffusion treatment (first thermal treatment) for about 100 minutes in a 1% O₂-containing nitrogen gas atmosphere at about 1100° C., whereby a p-type second semiconductor layer 2 b, which will be a channel forming region, is formed. Then, n-type impurity ions (ex. arsenic) are selectively implanted, followed by annealing treatment (second thermal treatment) for about 30 minutes in a 1% O₂-containing nitrogen atmosphere at about 950° C., whereby a third semiconductor layer 2 c to be a source region is formed.

[0075] In order to make such a structure function as FET, it is important that the second semiconductor layer 2 b and third semiconductor layer 2 c lie under the eaves of the trench gate 4 and are brought into contact with the gate insulating film 5 disposed in the trench. According to this invention, the first thermal treatment and second thermal treatment are conducted separately as described above for the control of the channel.

[0076] The deeper portion of the epitaxial layer 2 into which impurities have not been introduced, more specifically, the epitaxial layer 2 lying between the second semiconductor layer 2 b and semiconductor body 1, becomes the first semiconductor layer 2 a functioning as a drain region. The number of steps can be reduced by conducting ion implantation of the n⁺-type semiconductor region 12 a and the first semiconductor layer 2 a in the same step. This state is shown in FIGS. 19 and 20.

[0077] Since the second semiconductor layer 2 b, which will be a channel forming region, and the third semiconductor layer 2 c, which will be a source region, are formed by ion implantation while the upper surface of the trench gate 4 is positioned higher than the main surface of the semiconductor substrate, the profile in the depth direction in the semiconductor substrate 2 and the depth of each of the second semiconductor layer 2 b and third semiconductor layer 2 c can be controlled precisely, which makes it possible to accelerate thinning of the second semiconductor layer 2 b and third semiconductor layer 2 c. In short, the depth of the second semiconductor layer 2 b can be controlled precisely, whereby the channel length can be controlled precisely.

[0078] All over the main surface of the semiconductor substrate, an interlayer insulating film 7 is formed, for example, by depositing BPSG to give a film thickness of about 500 nm.

[0079] By anisotropic dry etching using a CHF₃ gas, contact holes CH are made in the interlayer insulating film 7 to expose the third semiconductor layer 2 c which will a source region, gate wiring 6, source guard ring semiconductor region 13 a and a connecting region of the protective diode 12. All over the main surface of the semiconductor substrate including the inside of each of the contact holes, a conductive film (metal film), for example, made of silicon-containing aluminum is formed. By patterning of the metal film, the gate guard ring 8, gate electrode 9, source wiring 10 and source guard ring 13 are formed, as illustrated in FIG. 21.

[0080] Conventionally, a contact layer 11 has been formed to extend from the main surface of the semiconductor substrate to the second semiconductor layer 2 b , and with this contact layer 11 and the third semiconductor layer 2 c around the contact layer 11, the source wiring 10 has been connected. In this embodiment, on the other hand, a contact hole CH is formed to extend to the second semiconductor layer 2 b by etching as illustrated in FIG. 22, followed by direct introduction of p-type impurities such as boron into the second semiconductor layer 2 b exposed by the contact hole CH as illustrated in FIG. 23. The p-type contact layer 11 is formed deeply by such a constitution, leading to a reduction in the generation amount of avalanche. Upon formation of the source, a mask for covering the contact layer 11 becomes unnecessary so that a subsequent photoresist step can be omitted. If the contact layer 11 is not required at the contact portion of another contact hole CH owing to the fabrication of the device into IC, it is possible to form a contact layer 11 only for a contact hole CH to be electrically connected easily with the source wiring 10 by using another mask covering the contact.

[0081] In this Embodiment, after introduction of impurities from the contact hole CH, silicon oxide of the interlayer insulating film 7 is selectively removed by etching relative to silicon on the main surface of the semiconductor substrate, whereby the surface of the third semiconductor layer 2 c is exposed by self alignment with the contact hole CH as illustrated in FIG. 24. Such a constitution makes it possible to enlarge the contact area of the third semiconductor layer 2 c with the source wiring 10 as illustrated in FIG. 25, thereby reducing the connection resistance.

[0082] In the next place, a protective insulating film 15 which covers the whole main surface of the semiconductor substrate is formed by applying and stacking polyimide onto a silicon oxide film obtained by plasma CVD with a tetraethoxysilane (TEOS) gas as a main source gas. In the resulting protective insulating film 15, a contact hole to expose each of the gate electrode 9 and the connecting region of the source wiring 10 is formed. The reverse side of the n⁺-type semiconductor body 1 is polished, followed by the formation thereover of a drain electrode 16 as illustrated in FIG. 4, for example, by successively depositing and stacking nickel, titanium, nickel and silver.

[0083] In this Embodiment, the p-type well 14 is disposed as a field relaxing portion in the form of a rectangular ring. Alternatively, it is possible to make a contact hole in the field insulating film 3 and introducing impurities from this contact hole to have the p-type wells 14 studded below the field insulating film in the ring form. In this constitution, the field relaxing portion can be formed after the formation of the gate wiring 6.

[0084] (Embodiment 2)

[0085]FIG. 26 illustrates another embodiment of the present invention.

[0086] In this embodiment, different from the above-described embodiment, the insulating film 17 and the field insulating film 3 are formed in the same step.

[0087] The fabricating method of the semiconductor device according to this embodiment will next be described based on FIG. 26.

[0088] Over an n⁺-semiconductor body 1, for example, made of single crystal silicon having arsenic (As) introduced therein, an n⁻-type epitaxial layer 2 having a lower concentration than the semiconductor body 1 is formed to give a thickness of about 5 μm by epitaxial growth. A silicon oxide film of about 600 μm thick is then formed, for example, by thermal oxidation over the main surface of the semiconductor substrate.

[0089] A mask is formed over the resulting silicon oxide film by photolithography. By etching through this mask, a field insulating film 3 having a rectangular portion inside of the corner is formed in the rectangular ring form along the outer periphery of the semiconductor substrate. The insulating film within a cell forming region surrounded by the field insulating film 3 is subjected to photolithography to form a resist mask having a pattern of a trench gate for a mesh gate structure wherein gates are two-dimensionally disposed in the lattice form. By etching through this resist mask, an insulating film 17 having a contact hole to expose the main surface of the semiconductor substrate according to the pattern is formed.

[0090] The steps subsequent thereto are substantially similar to those of the above-described embodiment so that the description is omitted.

[0091] This embodiment makes it possible to form the field insulating film 3 and the insulating film 17 in one step, thereby reducing the number of steps. Although the p-type well which will be a field relaxing portion is omitted in this embodiment, it can be formed as needed, for example, by making a contact hole in the field insulating film 3, introducing therefrom impurities and form p-type wells 14 studded in the ring form below the field insulating film.

[0092] The inventions made by present inventors have so far been described specifically based on the above-described embodiments. It should however be borne in mind that the present invention is not limited to or by them, but can be modified within an extent not departing from the scope of the present invention.

[0093] The present invention can be adapted, for example, to IGBT (Integrated Gate Bipolar Transistor), as well as power MISFET.

[0094] Advantages of the representative inventions, among the inventions disclosed by the present application, will next be described briefly.

[0095] (1) In the present invention, the upper surface of the trench-gate conductive layer is formed higher than the main surface of the semiconductor substrate, which brings about an advantage that a source offset can be prevented.

[0096] (2) In the present invention, the above-described advantage (1) makes it possible to promote thinning of a source.

[0097] (3) In the present invention, the above-described advantage (2) makes it possible to promote miniaturization of a cell.

[0098] (4) The present invention brings about an advantage that a trench gate conductive layer and gate insulating film can be formed over the main surface of the semiconductor substrate at the periphery of a trench wherein a trench gate is to be formed.

[0099] (5) According to the present invention, the above-described advantage (4) brings about another advantage that the damage of the gate insulating film can be prevented.

[0100] (6) According to the present invention, the channel forming region and source region can be formed to be shallow, because they are formed by thermal treatment control independently conducted after the formation of the trench gate. 

What is claimed is:
 1. A metal insulator semiconductor field effect type semiconductor device, comprising: a semiconductor substrate having first and second main surfaces which are opposite to each other; a trench formed in said semiconductor substrate to give a predetermined depth from the first main surface, said trench separating said first surface of said semiconductor substrate into plural semiconductor island regions for forming metal insulator semiconductor field effect type cells, respectively; a source region, a channel forming region and a drain region formed in the order of mention in the depth direction of said semiconductor island region from said first main surface of each said semiconductor island region; a gate insulating film formed over the inside surface of said trench, said gate insulating film having an end portion formed on said first main surface at the periphery of said trench; and a gate electrode formed over said gate insulating film so as to embed the trench therewith, said gate electrode being disposed over the end portion of said gate insulating film and extending from inside of said trench toward the periphery of said trench.
 2. A metal insulator semiconductor field effect type semiconductor device according to claim 1, wherein said gate electrode extends along said trench.
 3. A metal insulator semiconductor field effect type semiconductor device according to claim 1, wherein said gate electrode is made of polycrystalline silicon and said gate insulating film is made of silicon oxide.
 4. A metal insulator semiconductor field effect type semiconductor device, comprising: a semiconductor substrate having first and second main surfaces which are opposite to each other; a trench formed in said semiconductor substrate to give a predetermined depth from the first main surface, said trench separating said first main surface of said semiconductor substrate into plural semiconductor island regions for forming metal insulator semiconductor field effect type cells; a gate electrode formed, via an insulating film, to cover both the inside surface of said trench and said first main surface at the periphery of said trench; and a source region, a channel forming region and a drain region formed in the mention of order in the depth direction of each the semiconductor island region from the first main surface of the semiconductor island region.
 5. A method for fabricating a trench-gate type semiconductor device, comprising the steps of: providing a semiconductor substrate having a main surface, forming an insulating film over the main surface of said semiconductor substrate, patterning said insulating film into a pattern corresponding to a trench to be formed, etching the main surface of said semiconductor substrate by using said insulating film as a mask to form a trench in said semiconductor substrate, thereby defining a semiconductor island region, etching the side surface of the trench pattern of said insulating film to enlarge the trench pattern of said insulating film, thereby exposing the main surface of said semiconductor substrate at the periphery of said trench, forming a gate oxide film over the surfaces inside and at the periphery of the trench of said semiconductor substrate by using the insulating film as a mask, and forming a gate conductive layer over said gate insulating film inside and at the periphery of said trench.
 6. A method according to claim 5, wherein said gate conductive layer is made of polycrystalline silicon and said gate insulating film is made of silicon oxide.
 7. A method according to claim 5, further comprising the steps of, subsequent to the step for forming the gate conductive layer: introducing impurities into said semiconductor island region to form therein a channel forming region, and introducing impurities into said semiconductor island region to form therein a source region. 